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This standard is developed in accordance with the rules given in GB/T 1.1-2009.
This standard replaces GB/T 6398-2000 Standard Test Method for Fatigue Crack Growth Rates of Metallic Materials; the following technical changes have been made with respect to GB/T 6398-2000 (the previous edition):
——The application scope of the standard is modified (see Chapter 1);
——The "Normative References" is modified (see Chapter 2);
——The symbols and definitions are modified, which are divided into two chapters, i.e., terms anddefinitions as well as symbols and description (see Chapters 3 and 4; Chapter 3 of Edition 2000);
——The specimen type is modified (see Chapter 5; Chapter 4 of Edition 2000);
——The test equipment requirements are modified (see Chapter 6; Chapter 5 of Edition 2000);
——The test procedure requirements are modified; in this standard, the requirements of test process are stated in Chapter 7 "Test Procedure" and Chapter 8 "Crack Length Measurement";
——The requirements for test result treatment and calculation are modified (see Chapter 9; Chapter 7 and Chapter 8 of Edition 2000);
——The criteria for effectiveness test data, the criteria requirements for effectiveness test data of high-stress stiffening material and the content in the part of stress intensity factor calculation are deleted;
——The Annex A of the former standard is deleted; the Annex D of the former standard is adopted as Annex A (Informative) of this standard; the Annex E of the former standard is adopted as Annex D (Informative) of this standard; the Annex F of the former standard is adopted as Annex E (Informative) of this standard; the requirements of test procedure of fatigue crack growth rate in water-bearing media in Annex C (Informative) of this standard are added.
This standard has been redrafted and modified in relation to ISO 12108: 2012 Metallic Materials - Fatigue Testing - Fatigue Crack Growth Method. The main structures are consistent with that of the international standard. Modification and supplementation in the following aspects have been made in this standard with respect to ISO 12108: 2012, and the clauses modified or supplemented in main body have been marked with perpendicular single line at the margin:
——For normative references, this standard covers adjustment on technical differences so as to adapt to technical conditions in China, which is centralizedly reflected in Chapter 2 "Normative References"; specific adjustment is as follows:
GB/T 25917 modified in relation to international standard is used to replace ISO 4965.1 (see 6.1.1);
GB/T 10623 is cited (see Chapter 3);
GB/T 16825.1 is cited (see 6.1.3);
ISO 23788 is cited (see 6.1.2).
——The term and definition of precrack length are added (see 3.20);
——Error in ISO 12108: 2012 has been corrected, " is commonly defined as being the value of ΔK corresponding to a crack growth rate equal to mm/cycle" in ISO 12108: 2012 is changed as " is commonly defined as being the value of ΔK corresponding to a crack growth rate equal to mm/cycle" (see 9.3);
——In order to keep consistent with new international standard ISO 23788 Metallic Materials - Verification of the Alignment of Fatigue Testing Machines, the requirements of testing machine coaxiality are modified (see 6.1.2 and 5.1.2 of ISO 12108: 2012);
——According to calculation formula of loading coaxality in ISO 23788, the calculation formula of bending strain in international standard is deleted (see 6.1.2 and 5.4.5 of ISO 12108: 2012).
The following editorial changes are also made in this standard:
——The sequence of Chapter 5 "Apparatus" and Chapter 6 "Specimens" in ISO 12108: 2012 is adjusted;
——Annex B (informative) "Non-visual Crack Length Measurement Methodology - Compliance Method" is added;
——Annex C (informative) "Special Requirements for Determination of Fatigue Crack Growth in Water-bearing Media" is added;
——Annex D (informative) "Method for Determination of Small Fatigue Crack Growth" is added;
——Annex E (informative) "Method for Determination of Fatigue Crack Tension" is added;
——All formulae in the standard are numbered.
This standard was proposed by China Iron and Steel Association.
This standard is under the jurisdiction of SAC/TC 183 National Technical Committee on Iron and Steel of Standardization Administration of China.
The previous editions of this standard replaced by this standard are as follows:
——GB/T 6398-1986 and GB/T 6398-2000.
Metallic Materials - Fatigue Testing - Fatigue Crack Growth Method
1 Scope
This standard describes tests for determining the fatigue crack growth rate from the fatigue crack growth threshold stress-intensity factor range, , to the onset of rapid, unstable fracture.
This standard is primarily intended for use in evaluating isotropic metallic materials under predominantly linear-elastic stress conditions and with force applied only perpendicular to the crack plane (mode I stress condition), and with a constant stress ratio, R.
2 Normative References
The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
GB/T 10623 Metallic Material - Mechanical Testing – Vocabulary (GB/T 10623-2008; ISO 23718: 2007, MOD)
GB/T 16825.1 Verification of Static Uniaxial Testing Machines - Part 1: Tension/Compression Testing Machines - Verification and Calibration of the Force-measuring System (GB/T 16825.1-2008; ISO 7500-1: 2004, IDT)
GB/T 25917 Axial force-applied Fatigue Testing Machines Dynamic Force Calibration (GB/T 25917-2010; ISO 4965: 1979, MOD)
ISO 23788 Metallic Materials - Verification of the Alignment of Fatigue Testing Machines
3 Terms and Definitions
For the purposes of this document, the terms and definitions given in GB/T 10623 and the following apply.
3.1
crack length
a
linear measure of a principal planar dimension of a crack from a reference plane to the crack tip
Note: This is also called crack size.
3.2
cycle
N
smallest segment of a force-time or stress-time function which is repeated periodically
Note: The terms “fatigue cycle”, “force cycle” and “stress cycle” are used interchangeably. The letter N is used to represent the number of elapsed force cycles.
3.3
fatigue crack growth rate
da/dN
extension in crack length
3.4
maximum force
force having the highest algebraic value in the cycle; a tensile force being positive and a compressive force being negative
3.5
minimum force
force having the lowest algebraic value in the cycle; a tensile force being positive and a compressive force being negative
3.6
force range
the algebraic difference between the maximum and minimum forces in a cycle
3.7
force ratio
R
algebraic ratio of the minimum force to maximum force in a cycle
R=
Notes:
1 R is also called the stress ratio.
2 R may also be calculated using the values of stress-intensity factors: R= .
3.8
stress-intensity factor
K
magnitude of the ideal crack-tip stress field for the opening mode force application to a crack in a homogeneous, linear-elastically stressed body, where the opening mode of a crack corresponds to the force being applied to the body perpendicular to the crack faces only (mode I)
Note: The stress-intensity factor is a function of applied force, crack length, specimen size and geometry.
3.9
maximum stress-intensity factor
highest algebraic value of the stress-intensity factor in a cycle, corresponding to Fmax and current crack length
3.10
minimum stress-intensity factor
lowest algebraic value of the stress-intensity factor in a cycle, corresponding to Fmin and current crack length
Note: When R≥0, this definition remains the same, regardless of the minimum force being tensile or compressive. For R < 0, there is an alternate, commonly used definition for the minimum stress-intensity factor, = 0.
3.11
stress-intensity factor range
algebraic difference between the maximum and minimum stress-intensity factors in a cycle
Notes:
1 The force variables ΔK, R and are related as follows: .
2 For R ≤ 0 conditions, see 3.10 and 10.6.
3 When comparing data developed under R ≤ 0 conditions with data developed under R > 0 conditions, it may be beneficial to plot the da/dN data versus .
3.12
fatigue crack growth threshold
asymptotic value of ΔK for which da/dN approaches zero
Note: For most materials, the threshold is defined as the stress-intensity factor range corresponding to 10^(-7) mm/cycle. When reporting , the corresponding da/dN data used in its determination should also be included.
3.13
normalized K-gradient
C=(1/K)dK/da
fractional rate of change of K with increased crack length
C=1/K(dK/da)=1/Kmax (d Kmax /da)=1/ Kmin (d Kmin /da)=1/ Δ(dΔK /da)
3.14
K-decreasing test
test in which the value of the normalized K-gradient, C, is negative
Note: A K-decreasing test is conducted by reducing the stress-intensity factor either by continuously shedding or by a series of steps, as the crack grows.
3.15
K-increasing test
test in which the value of C is positive
Note: For standard specimens, a constant force amplitude results in a K-increasing test where the value of C is positive and increasing.
3.16
geometry function
g(a/W)
mathematical expression, based on experimental, numerical or analytical results, that relates the stress-intensity factor to force and crack length for a specific specimen configuration
3.17
crack-front curvature correction length
difference between the average through-thickness crack length and the corresponding crack length at the specimen faces during the test
3.18
fatigue crack length
length of the fatigue crack, as measured from the root of the machined notch
Note: See Figure 8.
3.19
notch length
the length from load line to root of the machined notch for CT or CCT specimen or from notch side edge to root of the machined notch for SENB or SENT specimen
3.20
precrack length
precrack length generated in fatigue loading and not participating in crack propagation rate calculation, which is used to eliminate the effects of machined notch on crack propagation rate
4 Symbols and Abbreviated Terms
4.1 Symbols
The symbols and designations used in this standard are shown in Table 1.
Table 1 Symbols and Their Designations
Symbol Designation Unit
Loading
C Normalized K-gradient
E Tensile modulus of elasticity MPa
F Force kN
Maximum force kN
Minimum force kN
Force range kN
K Stress-intensity factor MPa·
Maximum stress-intensity factor MPa·
Minimum stress-intensity factor MPa·
ΔK Stress-intensity factor range MPa·
Initial stress-intensity factor range MPa·
Fatigue crack growth threshold stress-intensity factor range MPa·
N Number of cycles —
R Force ratio or stress ratio —
Ultimate tensile strength at the test temperature MPa
0.2 % proof strength at the test temperature MPa
Geometry
a Crack length or size measured from the reference plane to the crack tip mm
Crack-front curvature correction length mm
Fatigue crack length measured from the notch root mm
Machined notch length Mm
Precrack length Mm
B Specimen thickness Mm
D Hole diameter for CT, SENT or CCT specimen, loading tup diameter for bend specimens Mm
g (a/W) Stress-intensity factor geometry function —
h Notch height mm
W Specimen width, distance from reference plane to edge of specimen mm
(W-a) Minimum uncracked ligament mm
Crack growth
da/dN Fatigue crack growth rate mm/cycle
Change in crack length, crack extension mm
4.2 Abbreviated Terms for Specimen Identification
CT Compact tension
CCT Centre cracked tension
SENT Single edge notch tension
SENB3 Three-point single edge notch bend
SENB4 Four-point single edge notch bend
SENB8 Eight-point single edge notch bend
5 Specimens
5.1 General
Proportional dimensions of six standard specimens: a compact tension (CT); a centre cracked tension (CCT) and three-, four- and eight-point single edge notch bends [(SEN B3), (SEN B4) and (SEN B8)]; and single edge notch tension (SENT) are presented in Figures 1~6, respectively. Machining tolerances and surface finishes are also given. A variety of specimen configurations is presented to accommodate the component geometry available and test environment and/or force application conditions during a test. The CT, SEN B3 and SEN B4 specimens are recommended for tension-tension test conditions only. The specimen shall have the same metallurgical structure as the material for which the crack growth rate is being determined. The test specimen shall be in the fully machined condition and in the final heat-treated state that the material will see in service.
Foreword i
1 Scope
2 Normative References
3 Terms and Definitions
4 Symbols and Abbreviated Terms
5 Specimens
6 Apparatus
7 Test Procedure
8 Crack Length Measurement
9 Calculations
10 Test report
Annex A (Informative) Non-visual Crack Length Measurement Methodology - Electric Potential Difference[4][6][14]
Annex B (Informative) Non-visual Crack Length Measurement Methodology - Compliance Method
Annex C (Informative) Special Requirements for Determination of Fatigue Crack Growth in Water-bearing Media
Annex D (Informative) Method for Determination of Small Fatigue Crack Growth
Annex E (Informative) Method for Determination of Fatigue Crack Tension
Bibliography
